SELECT PERSON_REF
FROM IMAGES
WHERE PERSON_REF IN (SELECT _PERSON_ID
FROM PERSONS
WHERE REGION_REF = (SELECT _REGION_ID
FROM REGIONS
WHERE REGION_ABB = "EU"))
GROUP BY PERSON_REF
HAVING Count(PERSON_REF) >= 3

this query gives my result like

2
4
5
6

Now I want to use that result in an other query, how to do that, Do I need to create loop or is there any other way

select Person_Ref from Images where Person_Ref IN (
Select _Person_ID from Persons where
Region_Ref in (select _Region_ID from Regions where Region_Abb = "EU"))
and EFFECT_REF = 2
group by Person_Ref HAVING COUNT(Person_Ref) >= 3

SELECT PERSON_REF
FROM IMAGES
WHERE EFFECT_REF = 2
AND PERSON_REF IN (SELECT PERSON_REF
FROM IMAGES
WHERE PERSON_REF IN (SELECT _PERSON_ID
FROM PERSONS
WHERE
REGION_REF = (SELECT _REGION_ID
FROM REGIONS
WHERE
REGION_ABB = "EU"))
GROUP BY PERSON_REF
HAVING Count(PERSON_REF) >= 3)

there are several ways you can do that.
You can use the IN example as was already ansered but it will work only if you need only one column (which is your case, I know, but it is good to know what you can do if you face a situation where you need more that oncolumn)

The simples would be wrap your query into parentesys and give it an alias so you can use it like if it were a table:

(SELECT PERSON_REF
FROM IMAGES
WHERE PERSON_REF IN (SELECT _PERSON_ID
FROM PERSONS
WHERE REGION_REF = (SELECT _REGION_ID
FROM REGIONS
WHERE REGION_ABB = "EU"))
GROUP BY PERSON_REF
HAVING Count(PERSON_REF) >= 3 ) MY_ALIAS

then yo ucan join with your other query. This is usually not a good approach though.

The best apporach, its to use Common Table Expressions (CTEs) so you would do something like this:

with MY_FIRST_QUERY as(
SELECT PERSON_REF
FROM IMAGES
WHERE PERSON_REF IN (SELECT _PERSON_ID
FROM PERSONS
WHERE REGION_REF = (SELECT _REGION_ID
FROM REGIONS
WHERE REGION_ABB = "EU"))
GROUP BY PERSON_REF
HAVING Count(PERSON_REF) >= 3
)
select MY_FIRST_QUERY.PERSON_REF

SELECT PERSON_REF
FROM IMAGES
WHERE PERSON_REF IN (SELECT _PERSON_ID
FROM PERSONS
WHERE REGION_REF = (SELECT _REGION_ID
FROM REGIONS
WHERE REGION_ABB = "EU"))
AND
EFFECT_REF=2
GROUP BY PERSON_REF
HAVING Count(PERSON_REF) >= 3